A large number of applications exist for a communications system where a central station periodically polls different ones of a number of outlying or remote receive/transmit stations for the purpose of supplying information to one or more of the outlying stations and for obtaining data from the outlying stations in response to a request for such data. Such systems are commonly known as "Polling" systems, and the messages directed to each of the remote stations are preceded by a unique address specifically identifying the station to which the following message is directed. Generally, at the end of each message, an end-of-message or end-of-transmission coded signal is sent to notify the receiving station, and all other stations on the system, that the particular message is terminated. Upon the termination of one message intended for one or more stations another message addressed to a different station or stations then is transmitted from the central station. Systems of this type often additionally include a transmission of the addresses of the various remote stations from the central polling station to initiate the transmission of desired data from the remote stations back to the central station. Upon receipt of its unique address, the remote station which has been polled by the central station then transmits data to the central station.
Such systems often are used for periodically monitoring remote unattended data collection stations, such as weather monitoring stations, remote pumping stations, and the like. In some situations, the remote stations are located in rugged terrain where one or more of the remote stations may be located relative to the central polling station such that radio communications obstruction is located in the transmission path between the remote station and the central polling station. Typically, obstructions are natural obstructions, such as mountains, or man made obstructions, such as high tension electrical lines and the like. In any event, the obstructions often make the communications path between the polling station and the remote station unreliable. In many situations, where a central polling station is located so that it has a clear radio communications path with all of the remote stations, subsequently-erected man-made structures may create a radio communications obstruction which did not originally exist when the system first was implemented.
In the past, it has been the practice, when a reliable direct communications link between the central polling station and one or more remote stations could not be maintained, to utilize specially constructed repeater stations located on the tops of hills or mountains or on top of tall buildings. These repeater stations are positioned so that a clear communications path exists between the central polling station and the repeater and between the remote station and the repeater. Repeater stations, particularly active repeater stations, are expensive since they must be separately and differently constructed apart from the central and remote stations in the system. Furthermore, particularly in the case of repeater stations located in rugged mountainous terrain, maintenance of such station is an additional ongoing added expense in the system with which they are used.
It is desirable to provide an inexpensive and flexible system for relaying or repeating messages for remote stations in a polling system whenever the direct communications link between a given remote station and the central polling station is unreliable.